1. Field of the Invention
The present invention relates to a signal detection method and apparatus and an optical recording and/or reproducing apparatus using the same, and more particularly, to a signal detection method and apparatus for performing an on-track control and/or detecting a track crossing direction, and an optical recording and/or reproducing apparatus using the same.
2. Description of the Related Art
An optical recording and/or reproducing apparatus performs a tracking operation to allow an optical pickup to accurately follow tracks on an optical information storage medium, for example, an optical disc.
A differential push-pull (DPP) method is used for tracking. In the DPP method, a tracking error signal is detected using a 4-division main photodetector and first and second 2-division sub photodetectors. An optical pickup used for an optical recording and/or reproducing apparatus that detects a tracking error signal using the DPP method radiates a main light beam M and first and second sub light beams S1 and S2 onto an optical disc 1, as shown in FIG. 1. The optical pickup includes a photodetector 3 which separately receives each of the main light beam M and the first and second sub light beams S1 and S2, which have been reflected from the optical disc 1, as shown in FIG. 2.
The photodetector 3 includes a 4-division main photodetector 5 which divides and receives the main light beam M, and first and second 2-division sub photodetectors 7a and 7b which divide and receive the first and second sub light beams S1 and S2, respectively.
Regardless of whether the optical disc 1 is a read-only memory (ROM) type or a land/groove type, because both an area 1a in which information is recorded and an area 1b in which information is not recorded can be regarded as tracks, hereinafter the area 1a in which information is recorded is referred to as a recording track and the area 1b in which information is not recorded is referred to as a non-recording track.
A track pitch “T/P” corresponds to the sum of a width of the recording track 1a and a width of the non-recording track 1b. When the optical disc 1 is a ROM type, the recording track 1a corresponds to an area with a sequence of pits, and the width of the recording track 1a corresponds to the width of a pit. The non-recording track 1b corresponds to an area located between adjacent sequences of pits in a radial (R) direction of the optical disc 1. When the optical disc 1 is a groove-only type, the recording track 1a corresponds to a groove, and the non-recording track 1b is a land.
When a main push-pull (MPP) signal is detected using a detection signal of the 4-division main photodetector 5 shown in FIG. 2, and a side push-pull (SPP) signal is detected using a detection signal of the first and second 2-division sub photodetector 7a and 7b, a DPP signal can be obtained by Formula (1).DPP signal=MPP−k1·SPP  (1)
Here, k1 denotes a gain adjustment constant. When a detection signal at each of the light receiving areas A, B, C, and D of the 4-division main photodetector 5, the light receiving areas E and F of the first 2-division sub photodetector 7a, and the light receiving areas G and H of the second 2-division sub photodetector 7b is denoted by the same reference character as the corresponding light receiving area, the main push-pull signal MPP and the side push-pull signal SPP are expressed by Formula (2), as is known widely.MPP=(A+B)−(C+D)SPP=(E−F)+(G−H)  (2)
Conventionally, an optical recording and/or reproducing apparatus compares the DPP signal with an operational signal R′X, which is obtained using a radio frequency (RF) signal RF′M of the main light beam M and an RF signal RF′S of the first and second sub light beams S1 and S2, and determines an on-track and a track-crossing direction, i.e., a seek direction.
According to conventional detection methods, the operational signal R′X is obtained by a signal detection apparatus shown in FIG. 3 using Formula (3).R′X=RF′M−k′·RF′S  (3)
The RF signal RF′M of the main light beam M corresponds to the sum of the detections signals at the four light receiving areas A, B, C, and D of the 4-division main photodetector 5 (RF′M=A+B+C+D). The RF signal RF′S of the first and second sub light beams S1 and S2 corresponds to the sum of the detections signals at the light receiving areas E, F, G, and H of the first and second 2-division sub photodetector 7a and 7b (RF′S=E+F+G+H).
As shown in FIG. 3 and Formula (3), conventionally, the operational signal R′X is calculated by a subtractor 9 which subtracts the RF signal RF′S of the sub light beams S1 and S2, whose gain k′ has been controlled by a gain adjuster 8, from the RF signal RF′M of the main light beam M.
When a CD-ROM optical disc having a storage capacity of 650 MB is used, the operational signal R′X and the DPP signal, which are obtained according to a conventional method, have different phase relations according to a seek direction, i.e., track crossing direction, as shown in FIGS. 4A and 4B. The phase of the operational signal R′X is different from that of the DPP signal by about ±90° according to the track crossing direction. Accordingly, it is possible to determine an on-track and the track crossing direction with respect to CD-ROM optical discs having a storage capacity of 650 MB using the operational signal R′X and the DPP signal which are obtained according to conventional methods.
More specifically, a CD-ROM optical disc having a storage capacity of 650 MB has a duty ratio of about 30%. The duty ratio indicates a ratio of the width of the recording track 1a to the width of the non-recording track 1b in a single track pitch T/P.
When the optical disc 1 has a duty ratio of about 30%, there is a phase difference of about 40-60° between a sum signal (A+B) of the detection signals at the two light receiving areas A and B located at one side of the 4-division main photodetector 5 and a sum signal (C+D) of the detection signals at the other two light receiving areas C and D. Similarly, the phase of the detection signal at the light receiving area E and G located at one side of the first and second 2-division sub photodetectors 7a and 7b, respectively, in the R direction is different from that of the detection signal at the light receiving areas F and H located at the other side of the first and second 2-division sub photodetectors 7a and 7b in the R direction.
As described above, because a phase difference exists between the sum signals (A+B) and (C+D) and between the detection signals E and G and the detection signals F and H, addition of these signals and subtraction of these signals give some values. Accordingly, when the optical disc 1 has a duty ratio of about 30%, on-track/off-track and a track crossing direction can be determined using the operational signal R′X and the DPP signal, which are obtained according to conventional detection methods.
When the duty ratio changes, however, a phase difference between the sum signals (A+B) and (C+D) also changes. In particular, when the optical disc 1 has a duty ratio of about 50% due to reduction of the width of the non-recording track 1b, the phase difference between the sum signals (A+B) and (C+D) would be about 180°. As a result, an appropriate operational signal R′X cannot be obtained with the conventional detection methods.
FIGS. 5A through 5C are graphs showing changes in a phase difference between the sum signals (A+B) and (C+D) depending on the duty ratio of an optical disc. FIG. 6 is a graph showing changes in the signal RF′M (=A+B+C+D), i.e., the sum of the sum signals (A+B) and (C+D), according to changes in a duty ratio of an optical disc.
FIGS. 5A through 5C show phase relations between the sum signals (A+B) and (C+D) generated when light is condensed by an objective lens having a numerical aperture of 0.5 and then radiated onto ROM type optical discs having a track pitch of 1.6 μm and different duty ratios. FIG. 6 shows changes in the signal RF′M according to a change in a duty ratio (pit width) when light is radiated onto ROM type optical discs under the same conditions described in FIGS. 5A through 5C.
Referring to FIG. 5A, when a pit on an optical disc has a width of 0.5 μm, that is, when the optical disc has a duty ratio of about 31%, a phase difference between the sum signals (A+B) and (C+D) is about 70°.
Referring to FIG. 5B, when a pit on an optical disc has a width of 0.6 μm, that is, when the optical disc has a duty ratio of about 38%, a phase difference between the sum signals (A+B) and (C+D) is about 120°.
Referring to FIG. 5C, when a pit on an optical disc has a width of 0.8 μm, that is, when the optical disc has a duty ratio of about 50%, a phase difference between the sum signals (A+B) and (C+D) is about 180°.
As described above, when the duty ratio is about 50%, a phase difference between the sum signal (A+B) and the sum signal (C+D) is about 180°, and therefore, a value of the signal RF′M obtained by summing the two sum signals (A+B) and (C+D) approximates to zero, as shown in FIG. 6. As a result, it is difficult to properly detect an operational signal used to determine on-track/off-track and/or a track cross direction with the conventional detection methods. In other words, when the duty ratio is 50%, it is impossible to detect an operational signal having sufficient phase and amplitude characteristics to determine on-track/off-track and a track cross direction with the conventional detection methods.
Meanwhile, a phase difference between the sum signals (A+B) and (C+D) changes depending on a track pitch, as shown in FIGS. 7A through 7D. FIGS. 7A through 7D show changes in phase difference between the sum signals (A+B) and (C+D) generated when optical discs have a duty ratio of 30% and different track pitches of 1.1 μm, 1.2 μm, 1.3 μm, and 1.6 μm, respectively. Referring to FIGS. 7A through 7D, a phase difference between the sum signals (A+B) and (C+D) changes from about 130° to about 90°, 80°, and 70° when the track pitch changes from 1.1 μm to 1.2 μm, 1.3 μm, and 1.6 μm. As shown in FIGS. 7A through 7D, the phase difference between the sum signals (A+B) and (C+D) also changes depending on the track pitch. Consequently, the operational signal R′X detected using conventional methods is influenced by the track pitch of an optical disc.
A light spot formed by radiating light onto an optical disc should have an appropriate size for signal detection, taking into account the track pitch of the optical disc. In other words, for successful signal detection, the light spot needs to have an appropriate size with respect to the track pitch. When considering such a relationship between the track pitch and the light spot, it can be inferred that the operational signal R′X detected using conventional methods is also influenced by the size of the light spot.
As described above, the operational signal R′X detected using conventional methods is influenced most by the duty ratio of an optical disc and also is influenced by the track pitch of the optical disc and the light spot size.
However, optical discs have been developed to, for example, have a high duty ratio by decreasing the width of the non-recording tracks 1b to meet demand for higher storage capacity. For example, conventional CDs usually had a storage capacity of 650 MB, but CDs have been developed to have a higher storage capacity by increasing the duty ratio. When the duty ratio of an optical disc is increased by reducing the width of the non-recording tracks 1b, the track pitch T/P decreases. As a result, the storage capacity can be increased.
Because the operational signal R′X detected using conventional methods is influenced by the duty ratio, the track pitch, and/or the light spot size, an optical recording and/or reproducing apparatus using conventional detection methods may not determine on-track/off-track and/or a track cross direction.